Pigment is traditionally dispersed into energy curable acrylated oligomers and monomers using of a 3-roll mill. The oligomers are very typically based upon epoxy or polyester chemistry, although other specialty oligomers are available. These are used to help disperse the pigment into the monomers and provide for good flow characteristics, although their viscosity is relatively high. The monomers can be mono-acrylates, di-acrylates or acrylates of higher functionality, but these are typically poor pigment dispersants. The monomers are used as carriers or diluents because they have much lower viscosity than the oligomers, and thus provide viscosity reduction, but they contribute to poor flow properties in highly pigmented dispersion bases.
The 3-roll mill process requires materials that have good flow characteristics. As a result, a high concentration of oligomer is necessary, while keeping monomer levels to a minimum. A typical dispersion base may contain 25%-40% pigment, 40%-50% oligomer and 10% 35% monomer, and typically will not be free flowing at room temperature. Commercial additives, such as those from Byk Chemie or Lubrizol (Solsperse), can be used to help improve flow in these bases. Additionally, Solsperse-type additives are utilized for pigment dispersion for UV ink jet applications, in which high-impact media milling is employed. This is described in U.S. Pat. No. 6,534,128 and WO 99/29788.
Grafted pigment additives such as those described in U.S. Pat. No. 6,284,816 have a significant effect on improving flow in pigment dispersion bases for energy curable systems. The advantage in using these additives is that higher pigment loading can be achieved with much improved flow. Oligomer content can be reduced relative to the pigment, while increasing the amount of monomer. The higher monomer levels do not pose flow problems, and in fact, they act as efficient viscosity reducers in these systems. These bases may be 3-roll milled at high pigment solids without detrimental effects. Additionally, these bases are amenable to high impact media milling. A typical dispersion base of this type will contain 30%-45% pigment, 3%-8% additive, 10%-30% oligomer and 17%-57% monomer. Dispersions produced with this approach are free-flowing at room temperature and have low viscosities.
Mixing pigment, a particle surface modifier (such as a polyolefin), oil and carrier resin followed by extrusion is described in WO 96/00261. This is then incorporated into various inks, including energy curable lithographic inks.
Flushers or attritors are well-known methods of dispersing pigment for conventional (i.e., non-energy curable) printing inks, such as making flush color for publication printing. The method of production of flush color involves removing water from a presscake in the presence of a hydrophobic resin. The water breaks away (separates) from the hydrophobic pigment and resin, and is removed. The pigment is dispersed into the resin by the kneading action of the blades. U.S. Pat. No. 6,316,517 describes flushing pigment presscake into a modified acrylate reactive oligomer for energy curable application. No additional additive technology is employed in this case.
Another approach is to use an attritor in combination with vacuum and heat to evaporate water from presscake, followed by kneading action to disperse pigment into a hydrophobic resin. Finally, dry pigment, that is a solid material, can also be employed in an attritor, where the shearing action of the kneader blades facilitate pigment dispersion.